The present invention relates to signal processing methods for use with infrared motion detection devices.
Infrared motion detection devices are commonly used in such applications as burglar alarm systems or automatic lighting devices. In the burglar alarm systems the device activates an alarm whenever an intruder moves into the monitored area. As part of an automatic lighting device the motion detection device causes a light to be turned on when a person or motor vehicle enters the area to be illuminated. Such devices may be used in residential lighting, for example, to illuminate a walkway as a person approaches the front door or to illuminate a driveway as a car approaches.
The devices function by sensing heat emitted from a person or other warm object such as an automobile as the person or object enters the field of view of the device. When the device detects an appropriate heat impulse, it provides an electrical signal to activate the light or other alarm. A problem arises, however, because the devices may also detect heat from any number of extraneous sources within the device's field of view, which could trigger a false alarm or turn on a light at an unwanted time. To counter this problem, infrared motion detection devices typically include signal processing circuitry for distinguishing or discriminating in some measure a characteristic of the signal expected from the desired target. For example, some detection devices include two or more separate detector elements which sequentially receive infrared radiation as an intruder or target object moves across the device's field of view. The included circuitry looks for a sequence of two or more corresponding pulses as the heat from the target object falls on the detectors. Circuitry of this type is disclosed, for example, in U.S. Pat. Nos. 3,760,399; 3,631,434; and 3,958,118. Another type of circuitry looks for a single pulse generated when heat from the intruder or target object impinges upon a single detector element. This type of circuitry seeks to discriminate against unwanted signals by responding only to pulses of a minimum threshold size. In this way the circuitry distinguishes pulses generated by weak incident infrared radiation, which is less likely to come from a human source or a motor vehicle. Circuitry of this type is disclosed, for example, in U.S. Pat. Nos. 3,703,718 and 3,928,843. In addition, such single-pulse circuitry may also include two or more detector elements connected in opposition to one another to discriminate against overall background changes in temperature. A temperature change over the area covered by the multiple detector elements produces opposite signals in opposing detector elements, which cancel one another and prevent the device from responding with an alarm.
Motion detection devices with the above signal processing circuitry nevertheless may still suffer from occasional false alarms or false triggerings.